Method for producing a porous titanium material article

ABSTRACT

Method for producing a porous titanium material for example a support. Starting from a titanium powder this powder is sintered under vacuum conditions in an inert/reducing atmosphere. Titanium hydride is added as powder and decomposes during sintering. The hydride ions provide a very reducing atmosphere preventing any titanium oxide or carbide/nitride composition to be formed at elevated temperature.

BACKGROUND OF THE INVENTION

The subject invention relates to a method for producing a poroustitanium material article. Porous means a porosity between 10 and 90vol. %.

Such an article can comprise both a 3-dimensional and a 2-dimensionalarticle. As example for a 2-dimensional product a support for aphotocatalyst is given or a product in which a large surface area isrequired. Other non exhaustive examples are electrodes, capacitors, fuelcells, electrolysers, structural parts and the like.

Processing massive titanium is generally known in the art and nodifficulties are encountered presently.

This is different for producing porous titanium. It is possible toobtain porous titanium which has however a very limited strength. In theabove applications high porosity, high surface area; corrosionresistance and weight are of importance as well as good mechanicalproperties.

In the prior art porous titanium has been produced by sintering titaniummetal powder. At elevated sintering temperature the titanium powder isvery sensitive to a clean atmosphere during processing. It has beenfound that titanium powder is very aggressive at elevated temperatureresulting in a surface layer for example a titanium oxide or titaniumcarbide layer. As soon as such a layer or an other layer is formedsintering is hampered because adhesion of adhesive powder particles isimpaired.

To solve this problem it is proposed in the prior art to add hydrogengas during sintering. In this way a reducing atmosphere can be obtained.However it has been found that even if hydrogen is added as a gas stillsintering of powder particles is far from optimum resulting in poormechanical properties of the final porous product.

U.S. Pat. No. 4,206,516 discloses a method for providing a poroussurface layer on a cast titanium substrate. To that end a slurry of puretitanium hydride is provided on the substrate. By thermal decompositiontitanium hydride particles convert in titanium metal. The slurry isprovided by spraying. Because pure titanium hydride particles are used,quite some shrinkage is to be expected after sintering.

U.S. Pat. No. 2,254,549 discloses a composition comprising 60–90% of abase metal not being titanium, a low melting temperature binder, whichcan comprise copper and titanium and metal hydride. The binder will bepresent in the final product.

U.S. Pat. No. 3,855,638 discloses a surgical prosthetic device whereon asolid metallic material substrate a porous coating is adhered. Thecoating is realized starting from an aqueous slurry which is dried andsintered in a hydrogen atmosphere.

U.S. Pat. No. 3,855,638 discloses a surgical prostetic device whereon asolid metallic material substrate a porous coating is adhered. Thecoating is realised starting from an aqueous slurry which is dried andsintered in a hydrogen atmosphere.

U.S. Pat. No. 3,950,166 discloses the use of either titanium or titaniumhydride and no mixtures thereof. The abstract of the Japanese patentspecification 2000-017301 discloses a sintered compact which is notporous because of a higher than 95% sintered density. A high percentage(35–95 wt %) titanium hydride powder is added to titanium powder.

U.S. Pat. No. 5,863,398 discloses a method for realising an object bysputtering.

The subject invention aims to provide an improved method for producing atitanium material article having increased mechanical properties.

SUMMARY OF THE INVENTION

Surprisingly it has been found that through the use of 0.01–10 wt %titanium hydride improved sintering characterisics and so improvedmechanical properties of the porous product are obtained. It is assumedthat this is caused by the fact that during the sintering processtitanium hydride decomposes at relatively low temperature and veryaggressive free hydride ions result adhering to any non-titaniumcomponent present at sintering. This prevents titanium compositions tobe formed at the surface of the titanium powder material so that a cleantitanium powder material is subjected to sintering at elevatedtemperature resulting in optimum sintering results.

Problems with shrinkage have not been observed. This means that thismethod is in particular useful for making two dimensional articles. Anexample is a support for a photo-catalyst and electro catalyst. Such asupport should have considerable mechanical strength and a high porosityat low thickness. As example a thickness between 50 μm en 2 mm ismentioned. This weight percentage is related to the total powdermaterial used during sintering.

Titanium hydride decomposes at relatively low temperature at about 288°C. and any contaminants present such as oxygen or carbon are interceptedby free hydrides (hydrogen ions) resulting. A further advantage of themethod according to the invention is that it is possible to keep thetemperature of sintering relatively low for example below 1000° C. Thesintering process lasts between 1 and 1000 minutes in particular about0.5–1 hour. It is possible with the method according to the invention toaccurately adjust the porosity of the product to be obtained.

According to a further preferred embodiment of the invention an organicbinder is provided which will evaporate during sintering or is fired inprevious step. As indicated above any carbon resulting having thetendency to react with titanium is catched away by hydrogen ions. Incontrast to metal binders such an organic binder is only used for givingshape to the article and is completely removed at sintering.

Vacuum is adjusted according to requirement and will be generallybetween 0.1 and 10 exp.(−6) atmosphere i.e. relatively low.

If 3D-articles are to be produced according to an embodiment of theinvention a foam is provided which is impregnated with the titaniummetal—titanium hydride powder after this powder is brought intosuspension. The foam is fired and the subsequent structure is subjectedto a sintering step. An other proposal is to subject the powder mixtureto a pressing step before sintering. This pressing step can be uni-axialor can comprise cold isostatic pressure. Preferably pure titanium (grade1–12) is used.

According to a further preferred embodiment the pressed article issintered on a substrate. Said substrate can comprise a molybdenum plate,which is coated with a (hexagonal) boron nitride spray for improvedadhesion. Other techniques for producing a sponge titanium structure arefeasible. For 2-dimensional products tape casting is a possibility.During tape casting a casting paste is produced from pure titaniumpowder, titanium hydride and an organic binder. Foil/tape are cast forexample with a doctor blade on a non-adhesive flat support such as aflat Teflon support. Subsequently the binder is removed by heating up to600° C. without the presence of oxygen. Carbon is made ineffective bythe effect of decomposing titanium hydride. Subsequently the foil/tapeis sintered in the presence of reducing agent.

The titanium material can be one of the materials as mentioned above.The organic binder can be an organic polymer binder such as polyvinylbutyral, meth-acrylate emulsion, etc. or one or more organic solvents(ethanol, isopropanol, toluene, terpineol etc.), organic dispersant(Menhaden oil, Corn oil, Glycerol trioleate, glycerol tristearate, oleicacid etc.), organic plasticiser (glycerine, dibuthyl phtalate,polyethylene glycol etc), release agent (stearic acid, etc), homogenizer(diethyl ether, cyclohexane, etc).

After preparing a foil/tape on a non-adhesive surface solvent it can bedried at room temperature in air and excess solvent can be removed. Thedry tape/foil can easily be removed from the supporting surface and cutto the required dimension. The mechanical strength is sufficient fortransferral. Subsequently the tape/foil is supported on a metal such asmolybdenum or tungsten coated with hexagonal BN suspension or zirconiapowders suspension and then heat-treated in a neutral atmosphere up to600° C. to pyrolyse all organic components. During this heating titaniumhydride and more particular hydride become effective. Subsequentlysintering is realised in a temperature range of 600–1600° C. in either aneutral atmosphere (argon, nitrogen) or a reducing atmosphere withhydrogen and an inert gas at more or less lowered pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be further elucidated referring to some examples.

I. In a First Example Dense 3D-Titanium Objects Such as Cylinders wereProduced.

Titanium powder (−325 mesh) was mixed with 7 wt % solution of PVApolymer (20 wt % concentration) and cylinders of 300 mm in diameter and10 mm high were pressed in an uniaxial press under a pressure of 100MPa. The samples were dried at the temperature of 80° C. for 2 h in anoven and then sintered in a vacuum oven on the molybdenum plate coatedwith a thin layer of hexagonal boron nitride. The sintering process wasperformed in a vacuum oven at 1300° C. for 2 h in the presence of theTiH₂ reducing agent in the quantity of 0.1 wt % to the total weight ofthe sample.

II. In a Further Example Porous 3-Dimensional Titanium Objects Such asCubes were Produced.

A 40 vol. % aqueous slurry of titanium powder was prepared using as rawmaterial the titanium powder (−325 mesh), water as a solvent and 5 wt %methylcellulose as a binder. The viscosity of the titanium slurry wasapproximately 2 cPa·s. The cubic shape samples of sizes 2.5×2.5×2.5 cm³from the polyurethane foam with 20 ppi were impregnated with the slurry.The excess of slurry was squeezed from the samples in a rolling press.The samples were dried at the temperature of 85° C. for 2 h in anelectrically heated oven and then sintered in a vacuum oven in thepresence of TiH₂ (reducing agent) at 1000° C. for 1 h. The shrinkage ofsamples was in the range of 15–16%, density of 0.45 g/cm³ and openporosity of 90 vol %.

III. In a Third Example a Porous 2-Dimensional Titanium Object wasProduced.

a) Preparation

-   -   Composition of the paste for tape casting:

titanium powder (−325 mesh) 55 wt % titanium hydrate 0.01 wt %   bindersystem B-33305 (from FERRO) 45 wt %(Polyvinyl Butyral based binder system using toluene/ethanol solvents;binder solids —22.4 wt %, resin/plasticizer ratio —1.7:1, Viscosity —450cPs).

-   -   All components of the paste were mixed by shaking in a Turbula        mixer for 45 min. and then tape casted on the glass plate coated        with Teflon tape. The viscosity of the binder system was        approximately 450 cPa·s. The doctor blade system was used for        forming a tape with the thickness of 0.5 mm and width of 30 cm.    -   The tape was dried in ambient atmosphere for 4 hours and then 1        hour in an oven at the temperature of 60° C.    -   The tape was cut for samples of sizes 12×12 cm². The samples        were located on the molybdenum plates coated with hexagonal BN        spray and then sintered in an electric oven between two Mo        plates separated by spacers under vacuum at a temperature of        1000° C. for 1 hour. The rate of heating: 200° C./h, rate of        cooling: together with the oven.

Although the invention has been elucidated above referring to preferredembodiments of the invention after the above description a personskilled in the art will immediately realise further embodiments whichare obvious after the above and within the range of the appended claims.

1. Method for producing a porous titanium material article, comprisingthe provision of titanium powder as base metal and titanium hydridepowder in a slurry as a powder mixture, sintering said powder mixture onat least 1000° C., said slurry comprising an organic binder comprisingan organic solvent, sintering is effected under vacuum conditions and inthat 0.01–0.10 wt % titanium hydride is provided.
 2. Method according toclaim 1, wherein said powder comprises an organic binder.
 3. Methodaccording to claim 1, wherein an organic foam is impregnated with saidpowder being brought into suspension.
 4. Method according to claim 1,wherein said powder is subjected to a pressing step before sintering. 5.Method according to claim 4, wherein the pressed article is sintered ona substrate.
 6. Method according to claim 5, wherein said substratecomprises a molybdenum substrate.
 7. Method according to claim 6,wherein said molybdenum substrate is coated with a hexagonal BN orzirconia layer.
 8. Method according to claim 1, wherein a titaniumpowder/organic binder paste is prepared and said powder is coated on asubstrate resulting in a combination.
 9. Method according to claim 8,wherein said combination is subjected to a heating step up to 1000° C.after which the substrate is removed and the foil/tape obtained issubjected to sintering.
 10. Method according to claim 8,comprising tapecasting.